Plasma widget settings dialog has badly rendered fonts

In the Plasma widget settings, the dialog window contains badly rendered fonts, as if scaled with the nearest neighbour method. This is not specific to any particular widget. For example, see the attached screenshot for Digital Clock settings (right-click on the Digital Clock widget on the panel, click on "Digital Clock Settings").

You can see on the screenshot that the fonts on the window title, buttons and other controls are rendered properly. So are the other fonts in the system, including KDE, Qt-based, and non-Qt-based applications. The problem is with the fonts used in the window contents. The fonts don't get fixed on window resizes or switching to another tab.

On this system, UI scaling is not used (i.e. the scaling ratio in KDE system settings is 1.0) but font DPI is increased (there is "Xft.dpi: 162" line in ~/.Xresources).

KDE bug: https://bugs.kde.org/show_bug.cgi?id=394044